The present invention relates generally to controlling the output of lights. More particularly, embodiments of the invention relate to a method and apparatus that use a shifting reference circuit for controlling light levels in an area or room.
Lighting control circuits are used with electronic dimming ballasts. These ballasts control the output of lights, such as fluorescent lights, that illuminate areas such as rooms, offices, patios, etc.
A conventional lighting control system measures the light in a separate environment outside the controlled area. Typically, a photocell is placed outdoors to detect sunlight. Such a system then uses information, e.g., illumination level, from the sunlight to adjust the light output in the controlled area. Such a system is called an open-loop system where the current ambient light level is not fed back into the system. Instead, an outside source alone, i.e., the sun, controls the system output. The sun, in effect, acts as a potentiometer controlling the lighting control system.
The design of these systems is based on the assumptions that the energy provided by the sun is proportional to visible light and that the light energy processed by the system directly represents visible light in the controlled area. Unfortunately, a system based on these assumptions results in inaccuracies. First, the sunlight""s total influence or contribution is great relative to its visible portion. Also, when sun goes up, the indoor lights dim with or without window coverings such as blinds, curtains, etc. Thus, if a sensor does not take into account the light provided by the ambient light in the area it controls, the actions of the system would be unpredictable, hence less useful.
Thus, it is desirable to have an alternative lighting control circuit that can distinguish between different light sources and control the lighting in a particular area accordingly.
The present invention achieves the above needs with a new lighting control circuit. More particularly, the present invention provides a lighting control circuit having a light sensor that outputs a first signal in response to being exposed to radiation. The lighting control circuit has a detection circuit that is coupled to the light sensor and is configured to generate a second signal from the first signal. The lighting control circuit has a driver circuit that is coupled to the detection circuit and is configured to generate a third signal to control an illumination level of a light, wherein an amplitude of the third signal is varied in response to the second signal and a reference signal. The lighting control circuit also has a shifting reference circuit configured to shift a reference voltage of the driver circuit to compensate for a supplemental sunlight energy contributed to the ambient light in a room.
In another embodiment, the driver circuit receives the second signal and compares it to the reference signal. Also, the driver circuit is configured to match a voltage level of the second signal to a voltage level of the reference signal via a feedback loop, thereby either raising or lowering the illumination level of a light until the voltage of the second signal matches that of the reference signal.
In another embodiment, the shifting reference circuit generates a correction voltage proportional to the supplemental sunlight energy contributed to the ambient light in a room and adds the correction voltage to the reference voltage in the driver circuit, thereby compensating for the supplemental sunlight energy.
In another embodiment, the feedback loop comprises an opto-electric path and an electronic path, the opto-electric path traveling from a light source controlled by the lighting control circuit to the light sensor via the radiation from the light, the electronic path traveling from the light sensor to the light source via the lighting control circuit.